1. Field of the Invention
The present invention generally relates to packaging of electronic circuits, especially integrated circuits having high heat dissipation requirements such as microprocessors and, more particularly, to the attachment of heat sinks to integrated circuit packages.
2. Description of the Prior Art
Heat dissipation is a major factor in the design of semiconductor devices such as analog and power transistors and especially in high performance digital switching circuits formed at high integration density. Ideally, in switching devices, heating is produced only during the switching transition interval and a small resistance in the "on" state. However, in high-performance circuits which provide very rapid switching transitions, it is generally the practice to exploit rapid switching by increasing the clock frequency so that the switching transition occupies a relatively constant portion of a clock period regardless of switching speed. Therefore, heat dissipation generally increases with clock speed.
Further, to exploit this increased clock frequency and to obtain reduced susceptibility to noise (as well as reduce manufacturing costs) there is substantial incentive to fabricate high-performance switching elements at the maximum possible integration density to minimize the length of signal propagation paths therebetween and to reduce noise susceptibility. Therefore heat dissipation requirements also increase proportionally with integration density.
Accordingly, it has recently become the practice to incorporate attachment of a heat sink or other heat removal structure (e.g. a liquid-cooled cold plate) into the design and manufacture of integrated circuit packages since heat removal is critical to both performance and reliability of the integrated circuit. In this regard, incorporation of the heat sink with the fabrication of the package is justified by the criticality of heat transfer from the package to the heat sink since uneven cooling may cause stresses within the chip or between the package and the heat sink. Thermal cycling can permanently damage the circuit elements (e.g. by diffusion or oxide growth) or connections formed on the chip (e.g. metal fatigue or migration) or degrade the attachment of the heat sink to the package which will then tend to increase the temperature excursion during thermal cycling.
For attachment of heat sinks to integrated circuit packages, it has been the practice to use an adhesive which has a relatively good thermal conductivity. However, the thermal conductivity of such materials is still very low compared to metals. For example, the thermal conductivity of a thermally conductive adhesive in current use is only about 1.73 W/m-.degree. C. whereas copper has a thermal conductivity of 395 W/m-.degree. C. Additionally, the interfaces of the package to the adhesive and the adhesive to the heat sink further impede heat transfer. Therefore, it can be understood that the adhesive connection of the heat sink is critical to both the thermal and electrical performance of the combination of chip, package and heat sink.
Specifically, the cross-section of the thermal path must be maximized and should not be compromised by gas or air bubbles. Such bubbles present a region of reduced thermal conductivity and two additional interfaces to impede heat flow. Further, thermal cycling causes expansion of the gas or increase of pressure within the bubble which can cause progressive breakage of the adhesive bond.
Additionally, it is known that a certain volume of adhesive is necessary to provide sufficient robustness of the bond to resist damage thereto by routine handling before or after the package is placed in service and the same applies to damage from gas or air bubbles, as well. On the other hand, since the thermally conductive adhesive has a significant thermal resistance, the length of the thermal path through the bond should be no more than required by the volume of adhesive necessary to a robust bond. Therefore, the thickness of the adhesive bond is relatively critical to the integrated circuit package.
It has therefore been the practice to bond heat sinks to integrated circuit packages with a reworkable thermoplastic adhesive which is initially in the form of a sheet of a thickness designed to provide the proper volume and thickness of the bond. In this sense, the sheet is essentially an adhesive preform and presents the problems of a requirement for heating the entire assembly to form the bond while heat transfer to the preform is low and irregular before the bond is formed and the possibility of capturing air or ambient gas at the surfaces of the sheet while the assembly is pressed together for heating and bonding. Throughput is low due to the thermal mass which must be heated and cooled.
A dispensable adhesive is an alternative to an adhesive preform. Unfortunately, dispensable adhesives do not fully solve the problems of a preform and present others. While air or gas will not generally be trapped by a dispensable adhesive that can flow when parts are pressed together, the thickness of the adhesive bond cannot be well-regulated. Further, good handling characteristics of dispensable adhesives such as ease of dispensing, long storage and pot life and short cure time generally imply poor thermal performance and viceversa. Poor thermal characteristics increase the criticality of the adhesive bond thickness.
Epoxies with suitable thermal conductivity, after mixing, must stay frozen until use, require special dispensing equipment, have a short working life and require a long oven cure. Suitable cyanoacrylate adhesives also require special dispensing equipment, the addition of an activator for curing and require only light handling, at most, for several hours after the bond is made. Either the long oven cure required by the epoxy or the period of restriction on handling of the device causes a restriction on the duration and throughput of the manufacturing process.
Accordingly, it can be seen that known alternatives for bonding heat sinks to circuit packages all present some unavoidable complexity in the manufacturing process and the possibility of compromising manufacturing yield or reliability.